Decoders of this type can conform to the "World System Teletext and Data Broadcasting System--Technical Specification", December 1987, published by the U.K. Department of Trade and Industry.
It is frequently necessary to determine when the transmission of a teletext page has been completed. For example, it may be desired to reallocate an acquisition circuit to a different page memory, or to commence processing of, so-called, extension packets. If these actions are commenced before a page has been completed an incorrect display may result. Conversely, if these actions are delayed by a significant amount a poorer performance will result; e.g. a delay in capturing a group of pages or a delay in generating the correct display.
Unfortunately there is nothing in the World System Teletext Technical Specification which guarantees the completion of the page. With the normal transmission sequence, a page commences with a page header, followed by the rows for the page. It might be though that detection of the following page header would be sufficient to indicate the completion of the page. However there are circumstances when this method cannot be relied upon.
For example, a problem can arise with the transmission of subtitles. Teletext subtitles for the deaf or in other languages are closely related to the accompanying picture signal and timing is quite critical if they are to appear and disappear in an appropriate way. It is therefore common practice for the transmission network to give a high priority for subtitles, in such a way that they can interrupt the transmission of a normal full teletext page. Thus, a page header for a normal teletext page may be transmitted followed by the rows of that particular teletext page. Before the normal teletext page has been completed, it may be decided to transmit a teletext subtitle and this is done by transmitting a subtitle page header followed by one or more teletext rows. The normal teletext page is then resumed by transmitting the page header for that page again followed by the remaining rows of that page. In this case the page header of the normal teletext page is followed by the subtitle page header, and this will be detected as an "end of page" signal which is produced before the normal teletext page has been completed.
A problem can also arise with the use of, so-called, "advanced headers". This is a transmission sequence used by some broadcasters to improve transmission efficiency. In the World System Teletext Technical Specification it is specified that the first transmission of a page header must be separated from the rows of the page by at least one field period in order to provide a page clearing interval which enables a previously acquired teletext page to be cleared from memory. If this does not occur naturally in the transmission sequence the conventional approach is to repeat the same page header or fill with other information until the next field interval. However this can waste a significant proportion of the transmission capacity if a large number of lines in the vertical blanking interval are being used. A way of minimizing this wastage is to use the "advanced header" technique whereby at the start of each page the current page header is preceded by the header for the next page in sequence. When the current page is completed, the page header for the next page is transmitted, and this can be followed immediately with the rows corresponding to that page, without the need to wait the one field period normally required for page clearing, since the "advanced header" has already given the warning necessary to fulfil the required page clearing requirements. Again, however, a simple "end of page" signal derived using the arrival of the next page header would result in an incorrect "end of page" signal being generated.
In order to overcome these problems and to take account of the "worst case" situation it has been found necessary to introduce a software delay, typically in the order of 0.5 seconds in order to determine whether an "end of page" signal is a true signal or not.